This invention relates generally to drilling. More specifically, the invention relates to cutter placement on a drill bit to minimize stick-slip occurrences.
Drill bits, particularly Polycrystalline Diamond Composite (PDC) drill bits commonly used for drilling earthen formations, typically exhibit blades of cutters that are nearly uniformly distributed around the bit axis of rotation. The cutters on the face of such drill bits will therefore evenly share the cutting load of the medium into which they are applied. In theory, by equally distributing the cutting load onto all cutters, the life of the drill bit should be maximized, with all bits wearing evenly until the end of their useful life.
However, such a symmetric drill bit may experience sticking and slipping of the drill bit in the medium when the drill bit either “bites” too much or too little of the medium to cause further boring into the medium. When the drill bit bites too much of the medium, each of the symmetrically cutters is engaging too much medium without enough torque to cause the medium to either be sheared or otherwise removed from the cavity. When the drill bit bites too little of the medium, the path of least resistance for the drill bits may be to skip over the medium rather than remove material from the cavity. Rippling caused by each symmetric cutter on the face of the bore hole may also result in each cutter arriving at the same time at the location of a ripple left by the previous cutter on the drill bit. This may happen repeatedly as the drill bit rotates against the face of the bore hole, causing both stick and slip conditions. Both stick and slip occurrences will cause torsional vibrations in the drillstring connecting the drill bit to drill's rotational power source.
Furthermore, because the drill bit is being urged into the medium during drilling, axial vibrations may occur from the drill bit being forced into a medium it is not actually cutting (sticking) or retreating from the medium, albeit slightly, when it is slipping. These torsional and axial vibrations can reduce the life of the drill bit and associated drilling equipment such as the rotational power source.